The structure for the 1 : 1 : 1 adduct formed by the reaction of triphenylphosphine with dimethyl acetylenedicarboxylate in the presence of carbon dioxide has been confirmed, and the adduct has been shown to dimerise with loss of carbon dioxide to give a stable alkylidenediphosphorane. The 31P and 1H n.m.r. and mass spectral data of the diphosphorane are discussed.